Cool Birds 1: the Hoatzin

December 1, 2005

Welcome to the first installment of a new feature for Feather Weather, a regular series on Cool Birds. These posts highlight birds which possess outstanding traits, sometimes beautiful, sometimes scary, sometimes just outright peculiar. These birds surprise and inspire us, and help us appreciate the grand biodiversity of our planet. Our inaugural feature today will be for the very distinctive Hoatzin, a bird that some consider the strangest bird in the world.

I was fortunate enough to go on a 5-day trip to the upper Amazon basin last May, specifically to Manu National Park in Peru. Tropical South America is the only place in the world where you can find the unique Hoatzin (pronounced “waat-zin”). This photo was taken by me on our last day in Manu, on Otorongo Lake (an oxbow lake near the river, a common place to find Hoatzin). We plied the calm lake waters in a small catamaran, and came across a half dozen individuals on a snag over the water. The other 4 left, but this character (and the one in the photo below) hung around to see what we were on about. I only had my Nikon point-n-shoot, but we got close enough for me to get this decent shot. It was a memorable moment, drifting as close as we did, all the while hearing the hiding birds perform their enigmatic wheezy whine calls.

The Hoatzin has a very primeval appearance – a blue patch of featherless facial skin, an upright spindly crest, and bright red eyes. The nestlings also possess something extremely rare among living birds – hooks at the elbow on either wing, to help them clamber back up logs or branches, in the event that they fall to the ground or into the water below. With age these hooks disappear, but their resemblance to fossilized features on Archaeopteryx adds to their prehistoric cachet.

The Hoatzin’s digestive system is also highly peculiar. Most birds have a system involving a crop, a gizzard, and the stomach – the Hoatzin’s crop instead has evolved into a foregut, making it more similar to a cow than to virtually any bird. (Only the Kakapo of New Zealand has something similar.) It needs this foregut because it digests leaves and other vegetable matter which require an area to ferment. The fermenting process takes on quite a stench though, and sure enough the bird is sometimes colloquially known as the “stinkbird”. This foregut adds considerably to the weight of the bird, making it something of a clumsy flier, which was apparent when we saw them flutter into the brush in Peru. (Fortunately we didn’t have to experience their stink!) It also means they have to spend some time digesting, which makes them relatively sedate while roosting and not so hard to see for interested birders. As to why the Hoatzin has this bizarre system, researchers remain uncertain.

The Hoatzin has also been difficult to discern taxonomically. Early systematists placed it among the Galliformes, which include birds like quail, grouse, turkeys, chickens, and pheasants. This made some sense, given the unclear relationships between these groupings of birds already – the Hoatzin was yet another mystery set among them. However, with more advanced biochemical and genetic methods, as well as continued anatomical and morphological study, the distinctiveness of the Hoatzin increased, and the closest relatives to the Hoatzin were determined to be the Cuculiformes, the Cuckoos. But including the Hoatzin among cuckoos is problematic not only because of the its aforementioned digestive system, but also unlike any cuckoo, the Hoatzin is anisodactyl, meaning that its feet have 3 toes forward and one pointing back, like many birds. All true cuckoos are zygodactyl, with 2 toes forward and two back. Thus, the Hoatzin is placed in a monotypic family (a family of one species) called Opisthocomidae. A few researchers then go further and place it in its own Order, Opisthocomiformes, while other more cautious types leave it among the Order Cuculiformes.

Regardless of whatever becomes known about the Hoatzin’s lineage, it certainly qualifies as a unique and remarkable creature. The Hoatzin is definitely a cool bird.